Magnetic resonance imaging (MRI) systems use a magnetic field to sense the structure of an object. The structure creates perturbations and variations in the strength in the imaging volume of the magnetic field. The perturbations and variations in the strength of the magnetic field are sensed and interpreted to provide an image of the structure of the object. In many MRI systems, the magnetic field is generated by a superconducting magnet to provide a strong magnetic field. The sensing and interpreting of the magnetic field assumes a magnetic field strength of a particular amount.
Unfortunately, superconducting magnets typically experience a small decay in field strength at a rate of the order of about 0.01 parts per million per hour. Field decay is also known as drift. For a preponderance of superconducting MRI systems, it is necessary to obtain a highly stable non-decaying field in order to properly interpret the magnetic field. Decay in the field strength contributes imprecision to the interpretation of the field, and ultimately, contributes error to the image produced from the magnetic field.
During NMR experiments, protons are detected by subjecting them to a large magnetic field which partially polarize the nuclear spins. The spins are then excited with radio frequency (RF) radiation, and, as they relax, they emit weak radio frequency radiation. The frequency of this radiation is proportional to the magnetic field to which they are subjected.
Spectroscopy and functional imaging (FMRI) are particularly sensitive to field decay. In spectroscopy, line widths approach 1 Hz. To improve the signal to noise ratio, hundreds of spectra, each spaced by a couple of seconds, are added together. Drift of more than 1 Hz over a few minutes, therefore results in a significant loss of resolution. fMRI images are also sensitive to frequency shifts of a few Hz. A 3 Tesla magnet decaying at 0.1 ppm per hour will see a field shift of 0.15 micro Tesla (6 Hz) during a 30 minute scan.
In most conventional MRI systems, the rate of decay is reduced to very low levels by manufacturing the MRI systems with high specification wire and sophisticated joints, which is expensive.
In some conventional MRI systems, the rate of decay is reduced to very low levels by including a secondary coil of appropriate geometry in the MRI system. These secondary coils are also known as drift compensation coils because the secondary coils counteract drift or decay in the strength of the magnetic field. As the current of a primary coil decays over time, the secondary coil conserves flux according to Lens' law and the secondary coil accumulates current. Thus the secondary coil maintains a very stable field in the imaging volume. Such secondary coils can be used to satisfy very low decay rate requirements or simply to retire the risk of field decay as a consequence of minor wire or joint defects in the primary coil. Drift compensation coils are also known as lock coils. Drift compensation coils are typically manufactured from a superconducting material.
Conventional drift compensation coils couple electromagnetically very strongly to the primary coil. Unfortunately, this coupling renders the drift compensation coils prone to extremely high peak induced currents (e.g. ˜1000 A) during a quench of the primary coil. For actively shielded magnets, this high current in a drift compensation coil can cause extensive magnetic field bloom which is potentially dangerous and could damage sensitive equipment in the vicinity of the magnet. Magnetic field bloom is an enlarged area of the magnetic field that can extend to areas where the magnetic field is harmful or dangerous to equipment or humans. Magnetic field bloom can cause electrical medical equipment to malfunction and cardiac pacemakers in humans to malfunction. Thus, conventional drift compensation coils can have harmful and dangerous effects that are contradictory to the goals of the healthcare settings in which they operate. Excessive peak induced current in the drift compensation coil may also damage the drift compensation coil.
In conventional MRI systems, the peak induced current caused by coupling between the drift compensation coils and the primary coil can be reduced by forcing the drift compensation coils to quench early. When the primary coil quenches, the drift compensation coil will rapidly accumulate current and, at some point, it will probably quench also because either it reaches the maximum current at which it can remain in a superconducting state, or an increasing Lorentz force produces a wire movement causing frictional heating.
In one example, early quench of the drift compensation coils is achieved by manufacturing the drift compensation coils with coil windings that have low critical current (Lc). Windings are also known as turns. In another example the drift compensation coils are forced to quench early by being fitted with quench heaters driven from the primary coil quench voltages.
Regardless of how early the drift compensation coil is quenched, these coils usually constitute a small number of windings. Thus, drift compensation coils have a low normal electrical resistance and therefore can continue to accumulate current after quenching. Increasing the number of turns increases the electrical resistance in the drift compensation coils and also increases the self inductance of the coil. Both factors tend to reduce the peak induced current in the drift compensation coils. However, inducing a quench of the drift compensation coil at the lower induced current is difficult. In addition, drift compensation coils with increased windings will cause increased fringe field contribution at any given current. If the quench simulations show that the coil continues to accumulate current, even after quenching itself, reducing the cross sectional area of copper in the wire will increase the electrical resistance and in turn reduce current accumulation in the drift compensation coil during a quench. However reducing the cross sectional area of copper in the wire will also reduce the normal current carrying capacity of the wire, resulting in increased risk of quench damage. The cross sectional area of copper in the conductor is the primary factor governing the magnitude of current the wire can carry when in a non-superconducting state.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a drift compensation coil in a MRI system that does not cause magnetic field bloom during a quench of the primary coil.